The purpose of pre-press proofs is to enable one to assess the color balance, registration, appearance, among other features, which can be expected from the press run and to correct the separation films before the printing plates are made therefrom. It is also desirable to produce so-called "customer proofs" which tell the customer how the original artwork will appear when printed with plates made from the separation films. Thus, it is essential that the pre-press proof have the same appearance as the press print. Accordingly, in addition to matching the color balance of the press print, the customer proof should be on the same paper as the press print.
The separation film can be a positive film or a negative film, depending on the type of printing plate to be used. The printing plate used can be the so-called positive working and negative working lithographic or offset printing plate as is known in this field. A positive working plate is exposed to light through a film positive on which the information to be printed corresponds to opaque areas and the non-printing background areas correspond to transparent areas. The exposed areas on the plate are rendered removable by chemical treatment and the underlying plate surface, usually grained aluminum, forms the water receptive non-printing or non-image areas, whereas the unexposed areas form the ink receptive printing image areas. A negative working printing plate is exposed to light through a film negative on which the information to be printed corresponds to transparent areas and the non-printing background areas correspond to opaque areas. In this case, the exposed areas on the plate become photo-hardened and form the ink receptive printing areas, whereas the unexposed areas are removed by chemical treatment and the underlying water receptive plate surface forms the non-printing or non-image areas.
It is also known to produce, by electrophotographic processes, lithographic and gravure pre-press proofs containing in general four colors, such as yellow, magenta, cyan and black. Such pre-press proofing processes are disclosed, for example, in U.S. Pat. Nos. 3,809,555 and 3,862,848. An apparatus for the production of electrophotographic pre-press proofs is described, for example, in U.S. Pat. Nos. 4,556,309 and 4,557,583.
It is known that electrophotographic pre-press proofs can be produced by charging a photoconductive recording member, followed by exposure through a separation film positive corresponding to one color, followed by toning of the exposed photoconductor with a liquid dispersed toner of the appropriate color, followed by in-register transfer of the color toned image deposit directly or through an intermediate or offset member to a receptor, such as paper usually of the same grade as the printing stock. These process steps are then repeated with separation film positives of the other three or more colors and appropriate color toners to produce a multicolor proof.
After all of the required color toner deposits have been transferred to the receptor paper, it is coated by spraying or other methods with a clear polymer layer to transparentize the color toner deposits and fuse them to the receptor paper sheet.
All of the above referred to prior art electrophotographic proofing processes are so-called direct reproduction processes. Accordingly, the color separation films employed can comprise film positives only, and thus, these processes are not suitable for the proofing of negative separation films wherein a reverse reproduction process is required.
Methods of electrophotographic image reversal, that is, production of a positive image from a negative film, are known, for example, as taught in U.S. Pat. No. 3,300,410 and United Kingdom Patent No. 998,599.
U.S. Pat. No. 3,300,410 discloses a photoconductive recording member that consists of a sheet of paper that is coated with photoconductive zinc oxide and charged to negative polarity. The sheet was exposed through a negative film and toned with a positive liquid toner having film forming colloidal size conductive resin particles to form, after evaporation of the carrier liquid of such toner and drying, a permanently fixed conductive and colorless film deposit in the unexposed or non-image areas. The sheet was then re-charged negatively and only image areas free of conductive colorless film deposit accepted charges. These areas were then toned with a colored positive toner to form visible image deposits, whereby a reversal image or a positive reproduction of the negative film was obtained. Since the conductive film deposit affixed in the non-image areas was colorless, it did not affect the appearance of the zinc oxide coating.
United Kingdom Patent No. 998,599 discloses an image reversal that was obtained on a sheet of paper coated with photoconductive zinc oxide in a similar manner as described above. However, a positive liquid toner comprising low tinting strength pigment particles was used to form, in the unexposed or non-image areas upon evaporation of the carrier liquid for such toner by drying, a permanently fixed conductive deposit. The deposit did not accept charge during the subsequent step of re-charging the surface for toning with a colored toner to form visible image deposits. Again, since the conductive deposit affixed in the non-image areas had a low tinting strength, it did not affect the appearance of the photoconductor. The low tinting strength materials used were alumina hydrate, magnesium and barium carbonates, talc, plaster of Paris, conductive zinc oxide, mica and silica, having a refractive index less than about 1.6 or 1.7 and an electrical volume resistivity less than about 10.sup.9 ohmcm.
In each of the above cases, the colorless or low tinting strength toner deposits were conductive and thus did not accept charges. Since these toner deposits were permanently affixed to the photoconductor surface, these processes are suitable only for single color reproduction on disposable photoconductors and are not suitable for applications wherein images are produced successively in a variety of colors on a reusable photoconductor and then transferred therefrom onto a receptor.